The human visual system perceives the high spatial frequencies of an image as sharpness and the low spatial frequencies as tonescale. That is, sharpens of an image is generally a function of how well the edges of objects in an image are perceived whereas the tonescale is a function of how the image's color and its gradual variation over the extent of the image are perceived. Since the human visual system perceives sharpened images much more favorably than unsharpened images, most electronic imaging systems which process recorded images attempt to provide sharpened images for human viewing.
In electronic imaging cameras, charge coupled devices (hereafter referred to as "CCDs") capture the image and introduce noise (hereafter referred to as "CCD noise") into the system. Typically, color processing circuitry then receives the recorded image with the inherent CCD noise and enhances and/or corrects the colors found in the recorded image. The color processing circuitry may transform the recorded image into a new tonescale and color coordinate system which is more suitable for processing by downstream circuitry and for eventual viewing. Unfortunately, the color processing circuitry also greatly amplifies the CCD noise.
A serially connected sharpening circuit then typically receives the color corrected recorded image in order to sharpen it. The sharpening circuit may receive the luminance components of the recorded image and sharpening algorithm. An example of such a mathematical sharpening algorithm is disclosed and explained in U.S. Pat. No. 4,941,190, entitled "Method and System for Enhancement of a Digitized Image", by Terrence H. Joyce which is incorporated herein by reference. Although different types of sharpening circuits attempt to differentiate between those high spatial frequency components attributable to noise and those attributable image information, such differentiation cannot be perfect and some portion of the high spatial frequency components of the noise is inevitably amplified. Thus, the sharpening as discussed above is always achieved at the expense of some increase in noise visibility. Therefore, the usefulness of sharpening circuitry is restricted by the amount of noise which becomes noticeably amplified after the recorded image is propagated therethrough.
As a result, a need exists in the art for an improved digital image processing circuit which does both color processing and image sharpening in a manner whereby the deleterious effects associated with the amplification of noise is minimized.